1. Field of the Invention
The present invention relates to a projection-type display apparatus such as a projector, and more specifically, to cooling of a reflection mirror for image formation of the projection-type display apparatus.
2. Description of the Related Art
Hitherto, for the purpose of reducing the size and thickness of a projection-type display apparatus, a rear projection display using an oblique projection system has been proposed (Japanese Patent Application Laid-Open No. 2001-255462). Specifically, the oblique projection system is a system such that a light projected from a lamp is sequentially reflected by a plurality of reflection mirrors and is projected onto a screen or the like placed ahead of the reflection mirrors. Along with the recent size reduction of an equipment and the increase in definition (display resolution) of an image, the reflection mirror has become an essential part which is required to have a high precision and a factor which may determine the comparative merits of a product.
However, the reflectance of the reflection mirror is not 100%, so that heat is held on a reflective surface of the reflection mirror although it is slight, and a temperature difference is generated between a reflective surface side and a rear surface side of the reflection mirror due to the influence of the heat, which results in a problem that the reflection mirror will be deformed due to a bimetallic effect.
In order to solve the above problem, a technology of sending air to a reflection mirror to thereby cool the reflection mirror with air has been proposed (Japanese Patent Application Laid-Open No. 2006-003541).
For example, in the case of cooling a reflective surface of a reflection mirror with air sent from a cooling fan so as to cool the reflection mirror, there is a problem that when the cooling fan is rotated at a high speed so as to obtain a sufficient cooling effect, the noise increases.
There is also another problem in terms of cost and power consumption, when the number of cooling fans is increased excessively in order that a plurality of cooling fans are rotated at a low speed to reduce the noise.